I think if you want to add a limit on use of cantrips as follows, it would be fine, and wouldn't change their use overly much:
"You can cast cantrips a number of times equal to your [spellcasting
ability score], until you are too mentally drained to cast a cantrip again without a short rest."
So most of the time, you can cast 16-18 cantrips until you need to rest an hour to be able to cast it anymore.
You can even add an optional grim and gritty version:
"You can cast cantrips a number of times equal to your [spellcasting
modifier], until you are too mentally drained to cast a cantrip again without a short rest. You can cast more than that without a rest, but each cantrip you cast beyond your [spellcasting
modifier] without a short rest, you must make a Constitution saving throw equal to your spellcasting DC for that spell, or else take 1 point of temporary Constitution damage."
That, or you can add a misfire chance to it after a certain point, or a fizzle chance after a certain point.
Edit: Moving to this thread instead.